Integrated circuit read only memory (ROM) devices are devices that contain a permanent set of information written into the memory. Under ordinary operation, the content of the memory remains unchanged. Such devices are useful in a variety of applications such as in microprocessor applications where permanent microprocessor instructions are stored within a ROM device.
At present, ROM devices ordinarily comprise an array of memory cells, where each memory cell contains information comprising one digital bit. The ROM array consists of several y-select lines, or columns, and several x-select lines, or rows. Each memory cell is connected between one of the y-select lines and one of the x-select lines, giving each cell a unique x,y address. In order to read the information therein, one of the y-select lines and one of the x-select lines are enabled or switched on, while the remainder remain switched off. When the enable lines are switched off, they are effectively open circuited. In this manner, only one memory cell is connected to an active circuit at a time.
In one prior art ROM circuit, each memory cell comprises a fusible link and a field effect transistor (FET), and is constructed in the following manner. The fusible link directly connects the y-select line to the drain of the FET. The gate of the FET is connected to the x-select line and the source is connected to ground. In order to program information into the ROM, select links are burned out.
The information stored within the ROM is read out in the following manner. A high logic voltage is first provided at the x-select line of the row of the cell to be read, charging the gate of each FET in the row. The y-select line then provides a high logic voltage to the column of the cell to be read, and further connects the column of the cell to be read to the ROM output. If the fusible link has been burned out, and the y-select line is open circuited, then the high logic voltage appears at the connection to the output. If the link has not been burned out, the FET provides a low impedance between the y-select line and ground because of the gate charge provided by the x-select line. As a result, the low impedance produces a low logic voltage at the connection to the output.
One disadvantage of this configuration is that because each cell includes a FET having three terminals, it requires three connections, one to the y-select line, one to the x-select line, and one to ground. Three connections is inefficient for a two dimensional memory.
While several other types of ROMs exist, most access and retrieve data in a similar manner, wherein a particular cell is activated or polled by turning on a select combination of an x-select line and a y-select line. One particular common aspect of existing ROMs is that a combination the x-select, y-select and memory cell FETs are used to connect a single memory cell to the ROM output at a time. Another common aspect is that an active element such as a switch or a FET is ordinarily associated with each memory cell.
It is often only necessary to read out the contents of a ROM in sequence. To do so, a source of read signals sequentially enables the x-select line and y-select line of each memory cell. A row, or x-select line is enabled while each of the y-select lines are enabled sequentially to read each memory cell within the row. When one row has been completely read, the next x-select line is enabled and the y-select lines are again enabled sequentially. The process is repeated for each row in the memory cell.
To turn on the y-select lines in sequence, a sequence of read signals have to be provided to the transistor switches of each y-select line. Each read signal turns on one FET, thereby connecting the y-select line to the ROM output long enough to read the contents of one memory cell. Thereafter, the read signal is removed and the next read signal is provided to the transistor switch of the next y-select line. As a result, the ROM must provide a sequence of read signals to each y-select line and then activate a transistor switch to connect the y-select line to the ROM output in order to read the contents of each cell.